RE: More than 24 bits of color

[Cathy Brown <catbrown@cwix.com>]

Thanks for the response, Russell. I knew something didn't make sense, but
didn't know why.

-----Original Message-----
From: owner-epson-inkjet@leben.com
[mailto:owner-epson-inkjet@leben.com]On Behalf Of Russell Williams
Sent: Monday, June 21, 1999 4:13 PM
To: epson-inkjet@leben.com
Subject: Re: More than 24 bits of color


Ian Lancaster@ibm.net wrote:

>Best not to acquire 16-bit images in Photoshop if you intend to covert them
>to 8-bit. The program (5.0.2) has a little known bug or "feature"
(depending
>on which way you look at it) which adds one RGB level of noise when
>converting from 16-bit to 8-bit space. In reality the conversion should be
>called 16-bit to 7-bit conversion.
>
>Bottom line in my view is: if you want to use 16-bit, then acquire from
>another application that supports it, such as Photopaint V8 which does a
>clean conversion to 8-bit.

I disagree. Photoshop's behavior is deliberate and preferable for almost
all uses, most especially for converting photographic scans from 16->8 bits
after adjustments and before printing.

What Photoshop does is add 1/2 lsb (least significant bit) of noise when
converting from 16->8 bits. This is the correct thing to do both
mathematically  and in terms of best image quality. If you don't do this, a
smooth gradient in 16 bit will band when converted to 8 bits. Other
programs may or may not do this. If they don't do noise dithering, your
results won't be "cleaner". Rather, they'll be more likely to show banding
-- some images will look subtly worse.

As a cautionary note, there's a guy who posts on the Photoshop newsgroup
named Timo who makes this "Photoshop's 16->8 bit conversion is buggy"
argument and who also advocates working in a gamma 1.0 space. I warn you
about him because his web pages about "how to calibrate your system" look
slick and believable, but if you follow his advice you'll get very bad
results (our tech support has to deal with his victims, guiding them
through recalibrating their systems).

Russell Williams
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Please: Stay on topic. Trim quoted messages.
http://www.leben.com/lists for list instructions.

-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Please: Stay on topic. Trim quoted messages.
http://www.leben.com/lists for list instructions.